Polyethylene is produced commercially in a gas phase reaction in the absence of solvents by employing selected chromium and titanium-containing catalysts under specific operating conditions in a fluid bed process. Polyethylene products of those original processes exhibited medium-to-broad molecular weight distribution. To be commercially useful in the gas phase fluid bed process, or a slurry reactor process, undertaken at low pressures, e.g. of less than about 1000 psi, the catalyst must exhibit high activity, with concomittant high catalyst productivity, because these process systems do not include catalyst residue removal procedures. Accordingly, catalyst residue in the polymer product must be so small that it can be left in the polymer without causing any undue problems in the fabrication and/or to the ultimate consumer. To this end, the patent literature is replete with developments of new catalysts.
The use of metallocene compounds of transition metals as catalysts for polymerization and copolymerization of ethylene is one of those developments. Metallocenes can be described by the empirical formula Cp.sub.m MA.sub.n B.sub.p. These compounds in combination with alumoxane have been used to produce olefin polymers and copolymers, such as ethylene and propylene homopolymers, ethylene-butene and ethylene-hexene copolymers, e.g., Kaminsky et al, U.S. Pat. No. 4,542,199 and Sinn et al, U.S. Pat. No. 4,404,344.
Methylalumoxane (MAO) is used as co-catalyst with metallocene catalysts. It belongs to the class of alumoxanes which comprises oligomeric linear and/or cyclic alkylalumoxanes represented by the formula:
R-(Al(R)-O).sub.n -AlR.sub.2 for oligomeric, linear alumoxanes and (-Al(R)-O-).sub.m for oligomeric cyclic alumoxane wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C.sub.1 -C.sub.8 alkyl group and preferably methyl. Methylalumoxane is commonly produced by reacting trimethylaluminum with water or with hydrated inorganic salts, such as CuSO.sub.4 5H.sub.2 O or Al.sub.2 (SO.sub.4).sub.3.5H.sub.2 O. Methylalumoxane can be also generated in situ in polymerization reactors by adding to them trimethylaluminum and water or water-containing inorganic salts. MAO is a mixture of oligomers with a very wide distribution of molecular weights and usually with an average molecular weight of about 1200. MAO is typically kept in solution in toluene. While the MAO solutions remain liquid at fluid bed reactor temperatures, the MAO itself is a solid at room temperature. PA1 R is CH.sub.3, C.sub.2 H.sub.5, n-C.sub.3 H.sub.7, i-C.sub.3 H.sub.7, n-C.sub.4 H.sub.9, i-C.sub.4 H.sub.9, t-C.sub.4 H.sub.9, C.sub.6 H.sub.5 or other alkyl substituted aryl group; PA1 R1=R2=H, CH.sub.3, C.sub.2 H.sub.5, n-C.sub.3 H.sub.7, i-C.sub.3 H.sub.7, n-C.sub.4 H.sub.9, i-C.sub.4 H.sub.9, t-C.sub.4 H.sub.9, C.sub.6 H.sub.5 or other alkyl substituted aryl group. PA1 (a) fluidizing a particulate, activated supported metallocene catalyst, in a fluid bed gas phase reactor, wherein the catalyst comprises a support which is silica, alumina or silica/alumina PA1 (b) contacting said catalyst with a feed selected from the group consisting of (1) ethylene, (2) ethylene admixed with hydrogen, (3) ethylene admixed with an alpha olefin of 3 to 10 carbon atoms, and (4) ethylene admixed with hydrogen and an alpha olefin of 3 to 10 carbon atoms; PA1 (c) maintaining a polymerization pressure of less than about 1000 psi, at a temperature which is in a range of from about 55 degrees C. as a lower limit and a maximum limit of about 115 degrees C.; PA1 (d) maintaining a polymerization temperature which is effective to control the molecular weight, expressed as MI, wherein MI decreases as the polymerization temperature is increased from said lower limit to said upper limit, and wherein MI increases as the polymerization temperature is decreased from said upper limit to said lower limit, PA1 (e) recovering polymerization product having a settled bulk density in the range of about 22 to about 36 lb/ft3 and a specific density of less than about 0.94 g/cc. PA1 R--(Al(R)--O).sub.n --AlR.sub.2 for oligomeric, linear alumoxanes and PA1 (--Al(R)--O--).sub.m for oligomeric cyclic alumoxane wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C.sub.1 -C.sub.8 alkyl group and preferably methyl. MAO is a mixture of oligomers with a very wide distribution of molecular weights and usually with an average molecular weight of about 1200. MAO is typically kept in solution in toluene.
Most of the experiments reported in the literature relating to methylalumoxane used as a cocatalyst with metallocene catalysts are undertaken in a slurry or solution process, rather than in a gas phase fluid bed reactor process.